Halo Instability in the Foreshock and Cosmic Ray Acceleration

A new mechanism of generation of compressional perturbations upstream of the Earth's quasi-parallel shock is proposed. In this region the ion distribution is characterized by two distinct populations, a core plasma of solar wind origin and a nearly isotropic, diffuse suprathermal ion distribution, the aureole or halo. Previously it was demonstrated that there is close correlation between the presence of diffuse upstream ions and the occurrence of hydromagnetic waves in the foreshock region but physical meaning of this interaction was not quite clear. It is found that relatively narrow halo can be unstable under excitation of fast magnetosonic waves. We term this new instability the halo instability. This instability has not been identified in the previous theoretical analyses and can serve as the dominant mechanism of the wave particle interactions in the foreshock region. A general dispersion relation for the halo instability as well as the compact expression for the maximum growth rate are derived. It is shown that broadening of the halo plays a stabilizing role and concrete conditions for such a suppression are found. The results of the theory are applied to existing satellite observations. It is shown that the physical mechanism of the halo instability has much in common with previously studied magneto-acoustic cyclotron instability that may lead to the generation of ULF waves with discrete spectra in the toroidal waveguide surrounding the Earth's magnetosphere.